The condensing mode of operation in gas phase polymerization reactors significantly increases the production rate or space time yield by providing extra heat-removal capacity through the evaporation of condensates in the cycle gas. Additional condensation is often promoted to extend the utility of condensed mode operation by adding an inert condensing agent (“ICA”) into the reactor. The most commonly used ICA's in commercial practice are n-pentane, isopentane, n-butane, isohexane, and isobutane. The amount of ICA that can be introduced into the reactor, however, must be kept below the “stickiness limit” beyond which the bed material becomes too sticky to discharge or to maintain a normal fluidization status. Running in excess of this limit will result in different types of fouling or sheeting in both type of fouling or sheeting and location in the reactor system. The primary limitation on increasing the reaction rate in a fluidized bed reactor is the rate at which heat can be removed from the polymerization zone. For example, in the commercial application of one of the most commons ICA's, isopentane, concentrations are pushed to the maximum allowable levels but no higher so as to avoid expanded dome section sheeting in a gas phase reactor. Past endeavors have attempted to improve on this technology by providing higher production rates for longer continuous run times.
For example, U.S. Pat. No. 5,352,749, is directed to a process for polymerizing alpha-olefin(s) in a gas phase reactor having a fluidized bed and a fluidizing medium wherein the fluidizing medium serves to control the cooling capacity of said reactor, the improvement comprising employing a level of liquid in the fluidizing medium entering the reactor which is in the range of from 17.4 to 50 weight percent based on the total weight of the fluidizing medium and maintaining the ratio of fluidized bulk density to settled bulk density above 0.59. Additionally, it is directed to a continuous process for increasing reactor productivity of a gas phase polymerization reactor having a fluidizing medium and a fluidized bed, said process comprising passing a gaseous stream comprising monomer through a reaction zone in the presence of a catalyst to produce a polymeric product, withdrawing said polymeric product, withdrawing said fluidizing medium comprising unreacted monomer from said reaction zone, mixing said fluidizing medium with hydrocarbon and polymerizable monomer(s) to form a liquid phase and a gas phase, and recycling said fluidizing medium to said reactor, the improvement comprising: a) introducing said hydrocarbon into said fluidizing medium to permit an increase in the cooling capacity of the fluidizing medium to a level in the range of from 42 Btu/lb to 110 Btu/lb; b) increasing the rate of withdrawal of polymer product to above 500 lb/hr-ft2; and c) maintaining the ratio of fluidized bulk density to settled bulk density above 0.59. A description of condensable fluids is provided in Col. 6, lines 31-47.
U.S. Pat. No. 6,063,877 is directed to a process for controlling a continuous gas phase exothermic process in a reactor having (1) a reactor inlet; (2) a reactor bed; (3) a reactor outlet; and (4) a recycle line with (a) a compressor and (b) a heat exchanger; with a pre-selected temperature for the reactor bed or outlet (2 or 3) and with a temperature differential (ΔT) between the temperature of the reactor inlet (1) and the pre-selected temperature of the reactor bed or outlet (2 or 3), comprising: (A) controlling the heat transfer provided by the heat exchanger (4(b)) to maintain the pre-selected reactor bed (2) or outlet (3) temperature, while simultaneously (B) controlling the feed rate to the reactor of a condensable fluid to maintain the temperature differential (ΔT) constant. Examples of condensable fluids are listed in Col. 2, lines 24-34.
U.S. Pat. No. 7,696,289 is directed to a gas phase polymerization process comprising the steps of: passing a recycle stream through a fluidized bed in a gas phase fluidized bed reactor, wherein the recycle stream comprises a low molecular weight dew point increasing component and a high molecular weight dew point increasing component; polymerizing at least one alpha-olefin monomer in the presence of a catalyst; and controlling an amount of the low molecular weight dew point increasing component in the recycle stream such that a dew point approach temperature of the recycle stream is less than the dew point approach temperature when operating with the higher molecular weight dew point increasing component alone. ICA's are described, for example, at Col. 15, lines 34-45, and claim 5.
U.S. Pat. No. 7,858,719 is directed to a gas phase process for polymerizing one or more hydrocarbon monomer(s) in a reactor in the presence of a catalyst system and a fluorinated hydrocarbon, where the fluorinated hydrocarbon is present at a partial pressure of 6.9 to 3448 kPa in the reactor and the reactor temperature is from 30 to 120° C., wherein the catalyst system comprises a Group 3 to 12 metal and the molar ratio of the fluorinated hydrocarbon to the metal of the catalyst system is from 2000-3500:1. A number of fluorinated hydrocarbons under the section header, “Condensable Fluids,” may be found in Col. 19 to Col. 21.
U.S. Patent Application Publication No. 2005/0182207 is directed to a continuous gas fluidized bed polymerization process for the production of a polymer from a monomer comprising: continuously passing a gaseous stream comprising the monomer through a fluidized bed reactor in the presence of a catalyst under reactive conditions; withdrawing a polymeric product and a stream comprising unreacted monomer gases; cooling said stream comprising unreacted monomer gases to form a mixture comprising a gas phase and a liquid phase and reintroducing said mixture into said reactor with sufficient additional monomer to replace that monomer polymerized and withdrawn as the product, wherein said liquid phase is vaporized, and wherein the stream comprises at least two inert condensing agents selected from the group consisting of alkanes, cycloalkanes, and mixtures thereof, each of the inert condensing agents having a normal boiling point less than 40° C. Table 1 provides a listing of ICA's.
Other background references include WO 94/28032, WO 2011/147539, and U.S. Pat. Nos. 6,262,192 and 7,683,140.
Despite these past endeavors, there is a need and desire to increase production rates while maintaining the continuity of the reactor system in a continuous process. Additionally, there is also a desire to broaden the polymer grade operating windows to produce polymers with different properties at higher production rates, for example, decreasing the density or raising the melt index of the polymer, which was not previously possible with current commercial practices due to the limitations of process conditions and readily available ICA's.